This invention relates generally to a lamp ballast output circuit, and more particularly to a lamp ballast output circuit having a series inductor-capacitor (L-C) resonant circuit operating substantially below the resonant frequency during pre-ignition of the lamp load.
In a conventional series connected L-C circuit, the lamp load is connected across the capacitor. During pre-ignition of the lamp load, the series L-C circuit operates substantially at its resonant frequency. That is, the driving signal applied to the series L-C circuit is at or near the resonant frequency of the series L-C circuit. In this way a sufficiently high pre-ignition voltage is applied across the lamp load for ignition of the latter.
The lamp load, typically of a fluorescent type, following ignition, achieves a substantially steady-state sinusoidal current flow therethrough by reducing the driving signal frequency well below the resonant frequency of the series L-C circuit. In determining when to switch from the resonant frequency to a different steady-state operating frequency, feedback circuitry is often required for sensing lamp ignition.
A sufficiently high voltage during pre-ignition of the lamp and sinusoidal lamp current following ignition (i.e. steady state operation), is commonly provided by a half-bridge inverter. The half-bridge inverter includes switching to control the frequency of the driving signal applied to the series L-C circuit. Control circuitry, responsive to the feedback circuitry, is required for controlling the speed at which the switching takes place.
Conventional lamp ballast output circuits, as described above, suffer from several drawbacks. For example, conventional lamp ballast output circuits require generating two different frequencies, that is, the resonant frequency during pre-ignition of the lamp load and a different steady-state operating frequency. Such circuits also require sensing circuitry to determine when to switch from the resonant frequency to the steady state operating frequency.
It is particularly undesirable to operate at or near the resonant frequency of the series L-C circuit before lamp ignition inasmuch as unsafe, high voltages and current levels can occur (i.e. above the maximum ratings of one or more ballast circuit components). By operating below resonance during pre-ignition of the lamp load, capacitive switching of the half-bridge inverter can easily occur producing high switching losses. Additional circuitry is therefore required to prevent the half-bridge inverter from operating below the series L-C circuit resonant frequency during pre-ignition of the lamp load.
The inductance of inductor L is normally determined based on the desired lamp current during steady state conditions. The capacitance of capacitor C is thereafter chosen so as to provide a resonant condition (typically between 20-50 kHz for a fluorescent lamp). Generally, the capacitance of capacitor C is between about 5 to 10 nanofarads leading (with the additional high voltage capability) to a relatively costly capacitor requiring a relatively large space on a printed circuit board.
Accordingly, it is desirable to provide a lamp ballast output circuit having a safe open circuit (i.e., pre-ignition) voltage and current level, with relatively low switching losses. The improved lamp ballast output circuit should not need a driving signal at more than one frequency, this frequency being well below resonance of the series L-C circuit. It is also desirable that the improved lamp ballast output circuit permit use of a relatively less expensive, smaller capacitor in order to lower the lamp ballast manufacturing cost and to reduce the reactive current flowing through the capacitor after lamp ignition thus lowering circuit power loss.